1. Field of the Invention (Technical Field)
The present invention relates to tone burst spectroscopy and to diode laser spectroscopy.
2. Background Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Absorption spectrometry is a widely used method for measuring the presence or concentration of chemical compounds. Modulation techniques have been widely used to improve the sensitivity of absorption spectrometers. In these techniques, the optical frequency or wavelength of a light source is rapidly varied, and the output of the spectrometer is analyzed in a way that exploits the wavelength dependence of the compound under study. Modulation techniques improve sensitivity, in part because the lasers that are used as light sources typically have less noise at high frequencies. Such noise typically follows a 1/f distribution, where f is the frequency. In addition to tone burst spectroscopy, modulation techniques include wavelength modulation, frequency modulation, and two-tone frequency modulation. These other modulation techniques have been described in many publications, for example Pavone et al., xe2x80x9cFrequency- and wavelength-modulation spectroscopies: comparison of experimental methods using an AlGaAs diode laser,xe2x80x9d Applied Physics B, 56, 118-122 (1993); J. A. Silver, xe2x80x9cFrequency Modulation Spectroscopy for Trace Species Detection: Theory and Comparison Among Experimental Methods,xe2x80x9d Applied Optics 31, 707-717 (1991); P. Kluczynski et al., xe2x80x9cTheoretical description based on Fourier analysis of wavelength-modulation spectrometry in terms of analytical and background signals,xe2x80x9d Applied Optics 38, 5803-5815 (September 1999); G. Bjorklund, Method and Device for Detecting a Specific Spectral Feature, U.S. Pat. No. 4,297,035 (1981); and T. Gallagher et al., Frequency Modulation Spectroscopy Using Dual Frequency Modulation and Detection, U.S. Pat. No. 4,765,736 (1988). Frequency modulation and wavelength modulation methods effectively avoid laser 1/f noise, but they introduce strong amplitude modulation of the laser at the modulation frequency. This amplitude modulation can be 10% of the laser power and may differ from the detection frequency by a factor of two, making it difficult to remove by filtering. Two tone modulation avoids both laser 1/f noise and amplitude modulation at frequencies near the detection frequency, but at a cost of a lower theoretical signal to noise ratio.
In the tone burst method, the light from the laser (or other narrow band source) is transmitted through a sample and on to a detector that produces an electrical output proportional to the transmitted power. The laser""s optical frequency or wavelength is modulated at a tone frequency F1 and the modulation is turned on and off at some lower, burst frequency F2. The output of the photodetector, suitably amplified, is measured with a lock-in amplifier referenced to the frequency F2. The output of the lock-in is a measure of the difference in transmission with and without the modulation. A strong signal is produced when the laser wavelength is near the wavelength of a narrow spectral transition. Detection at F2 avoids laser excess noise at still lower frequencies. Tone burst spectroscopy thus provides a practical method for sensitive detection of small, wavelength dependent absorbances.
Tone burst spectroscopy is effective because the tone modulation spreads the laser output across a range of wavelengths that is wider than the absorption feature being examined. If the average (i.e., unmodulated) laser wavelength is nearly coincident with the center of an absorption feature then more light is transmitted through the sample when the tone modulation is on than when the tone modulation is off. In other words, when the tone modulation is on, some of the laser output is shifted away from the center of the absorption feature to the wings of the absorption feature where the absorption is weaker. Switching the tone modulation on and off at the burst frequency produces a synchronous change in the amount of light that is transmitted through the sample to the detector. Thus, the detector output also changes synchronously with the burst modulation and the extent of change is a measure of the optical absorbance.
Tone burst spectroscopy has been developed by a number of workers. H. M. Pickett, xe2x80x9cDetermination of collisional linewidths and shifts by a convolution method,xe2x80x9d Applied Optics 19, 2745-2749 (1980), first applied the tone burst method to absorption spectroscopy in microwave experlements. C. S. Gudeman et al., xe2x80x9cTone-burst Modulated Color-center-laser Spectroscopy,xe2x80x9d Optics Letters, Vol. 8. pp. 310-312 (1983). demonstrated the use of lone burst spectroscopy with a color center laser. To modulate the frequency of the laser, an electro-optic modulator was used. The tone burst modulation waveform was generated from two oscillators, the F1 oscillator operating at 400 MHz and the F2 oscillator at 10-100 kHz. these oscillators were combined in a radio frequency mixer, amplified, and used to drive the electro-optic modulator crystal. Problems that arose included reflections within the optical system and overheating of the modulator. H. Adams et al., xe2x80x9cSensitivity Improvement of Tone-burst Modulated Spectroscopy with a Color-center Laser,xe2x80x9d J. Opt. Soc. Amer. B (1984) Vol. 1, No. 5, pp. 710-714, improved the tone burst method with a specially designed electro-optic modulator which was wedged to reduce the effects of reflections and water cooled to prevent overheating. The tone frequency (200-300 MHz) was generated by an rf generator, while the burst frequency was generated using a separate square wave generator at 10-100 kHz.
M. C. Chan et al., xe2x80x9cLaser Spectroscopic Studies of the Pure Rotational U0(0) and W0(0) Transitions of Solid Parahydrogen,xe2x80x9d J. Chem. Phys. Vol. 95, No. 1, pp. 88-97 (1991), demonstrated the use of tone burst detection on solid samples. The tone frequency was 40 MHz, the burst frequency was 6 kHz, and the two oscillators used to produce these frequencies were coupled using a double-balanced mixer.
H. Sassada et al., xe2x80x9cTi-Sapphire Laser Spectrometer for Doppler-limited Molecular-Spectroscopy,xe2x80x9d J. Opt. Soc. Amer. B Vol. 11, pp. 191-197 (1994), notes the close connection between tone burst spectroscopy and two-tone frequency modulation spectroscopy. Two oscillators are used, the tone at 196 MHz and the burst at 930 kHz. The burst oscillator is used to drive an RF switch. The switched output is amplified and coupled to an electro-optic modulator.
In implementing tone burst detection, there are some constraints on the frequencies of the tone (F1) and the burst (F2). Previous work has used tone frequencies F1 that are comparable to or greater than the line width of the spectral transition to be measured. This corresponds to the frequency modulation regime in frequency modulation spectroscopy (see Silver). The modulation bandwidth of the light source sets a practical upper limit on the modulation frequency, which may make it difficult to measure broad transitions using tone burst in the frequency modulation limit. The burst frequency F2 must be less than half the tone frequency F1 and must be within the bandwidth of the detector and lock-in amplifier. Typically, the burst frequency is a small fraction of the tone frequency, e.g., the burst frequency was approximately {fraction (1/200)} the tone frequency in the work by Sassada.
In the studies cited above, the tone frequency F1 was much greater than the burst frequency F2. As a result, any amplitude modulation of the laser at the tone frequency is easily suppressed by the filters associated with the detection circuitry. However, the use of higher modulation frequencies demands a higher degree of care in designing electrical circuitry to generate and control the modulation. Changes in reactance can change the modulation amplitude, altering the calibration of the spectrometer. Thus it may be desirable to employ a lower value for the tone frequency. When the tone frequency is close to the burst frequency, amplitude modulation at the tone frequency may cause an unwanted signal at the burst frequency. The magnitude and phase of this interfering signal will depend on the exact frequency and phase relationship of the tone and burst frequencies. If these frequencies are generated independently from separate oscillators, then the phase and amplitude of the interference may drift due to variations in the frequencies of these oscillators.
Lock-in amplifiers or phase sensitive detectors are used to recover the signal at the burst frequency. These have a signal input and a reference input that defines the measurement frequency. The reference input usually is converted to a calibrated amplitude, but it can have an adjustable phase. The lock-in amplifier signal input for tone burst is the output of the photodetector after any preampliers, and the reference input is the burst frequency F2. The lock-in amplifier output is usually a voltage or a digitized value that indicates the amplitude of the signal at the frequency F2. There are many different circuit implementations of a lock-in, including double balanced mixers, switched capacitor filters, transistor multiplier circuits and digital signal processors that take the product of the digitized input signal times a reference waveform. These implementations vary in cost, sensitivity, size, power consumption, and other features that may affect the selection for a particular application. However, it is important to distinguish between two classes of lock-in amplifiers based on their function. The xe2x80x9cTrue sine wavexe2x80x9d lock-in has an output that is proportional only to the sinusoidal part of the input signal at the reference frequency. Mathematically, the output is proportional to the signal input times a sine wave at the reference frequency. The xe2x80x9csquare wavexe2x80x9d lock-in has an output that is proportional to the signal input times a symmetrical square wave at the reference frequency (N. Goldstein et al., xe2x80x9cMeasurement of molecular concentrations and line parameters using line-locked second harmonic spectroscopy with an AlGaAs diode laser,xe2x80x9d Applied Optics 31, 3409-3415 (1992). In either the square wave or sine wave lock-in, the output depends further on the reference phase angle. It is also possible to measure the signal amplitude at the frequency F2 by phase-less techniques such as can be provided by a quadrature phase lock-in or by suitable filters and an ac voltmeter.
D. C. Look et al., Apparatus and Method for Measuring Nuclear Spin-lattice Relaxation Time (T1) by Tone-burst Modulation, U.S. Pat. No. 3,568,047, describe the use of tone burst modulation in nuclear magnetic resonance spectroscopy. A magnetic field modulation, either sinusoidal or triangular, is applied to the sample using of a coil. The frequency ratio of the tone relative to the burst is controlled, so that the burst contains a precise number of tone periods with a constant phase. The tone burst is also used to synchronize an oscilloscope, and an oscillogram of the magnetization induced in the sample is recorded and analyzed to determine the spin-lattice relation time.
The present invention provides a means to stabilize the phase relationship between the tone and burst frequencies so that the effects of amplitude modulation at the tone frequency are constant in time. It further provides a method to reduce the interference by choosing the number of modulation periods contained within the burst. These improvements permit the use of lower tone frequencies or higher burst frequencies without compromising the sensitivity of the measurement. The invention further provides for a variety of tone modulation waveforms.
The present invention is of a tone burst spectrometer and spectrometry method comprising: providing a semiconductor laser with an input controlling operating wavelength and a light output; passing the light through an area containing a sample; detecting the light with a detector; generating a tone burst modulation waveform via a generator connected to the wavelength control input of the semiconductor laser; and synchronizing a lock-in amplifier connected to an output of the detector to a burst frequency of the generator.
The present invention is also of a tone burst spectrometer and spectrometry method comprising: providing a narrow bandwidth light source with an input controlling operating wavelength and a light output; passing the light through an area containing a sample; detecting the light with a detector; generating a tone burst modulation waveform via a generator connected to the wavelength control input of the semiconductor laser; and synchronizing a lock-in amplifier connected to an output of the detector to a burst frequency of the generator; and wherein the tone frequency of the generator is an integer multiple of the burst frequency. In the preferred embodiment, the tone frequency is either an odd integer multiple of the burst frequency and the lock-in amplifier is a xe2x80x9csine wavexe2x80x9d type lock-in amplifier, or a multiple of four times the burst frequency and the lock-in amplifier is a xe2x80x9csquare wavexe2x80x9d type lock-in amplifier.
The present invention is further of a tone burst spectrometer and spectrometry method comprising: providing a narrow bandwidth light source with an input controlling operating wavelength and a light output; passing the light through an area containing a sample; detecting the light with a detector; generating a tone burst modulation waveform via a generator connected to the wavelength control input of the semiconductor laser; and synchronizing a lock-in amplifier connected to an output of the detector to a burst frequency of the generator; and wherein the tone modulation waveform is other than a sine wave. In the preferred embodiment, the tone modulation waveforms a square wave or a triangle wave. The tone modulation waveform preferably comprises a filtered noise waveform produced by high pass filtering a white noise source to remove noise components at the burst frequency and lower frequencies.
The present invention is additionally of a tone burst spectrometer and spectrometry method comprising: providing a narrow bandwidth light source with an input controlling operating wavelength and a light output; passing the light through an area containing a sample; detecting the light with a detector; generating a tone burst modulation waveform via a generator connected to the wavelength control input of the semiconductor laser; and synchronizing a lock-in amplifier connected to an output of the detector to a burst frequency of the generator; and wherein the tone frequency of the generator is less than a half width at half maximum of a targeted spectral transition.
The present invention is also of a tone burst spectrometer and spectrometry method comprising: providing a narrow bandwidth light source with an input controlling operating wavelength and a light output; passing the light through an area containing a sample in sufficient concentration to produce a spectral feature with a high signal to noise ratio; detecting the light with a detector; generating a tone burst modulation waveform via a generator connected to the wavelength control input of the light source; and synchronizing a lock-in amplifier connected to an output of the detector to a tone frequency of the generator or an odd harmonic of the tone frequency; and wherein the output of the lock-in amplifier is low pass filtered and negative feedback is employed to stabilize the operating wavelength at or near a center wavelength of a spectral feature of the sample.
A primary object of the present invention is to stabilize the phase relationship between the tone and burst frequencies so that the effects of amplitude modulation at the tone frequency are constant in time.
Another object of the invention is to reduce interference by choosing the number of modulation periods contained within a tone burst.
A primary advantage of the present invention is that it permits the use of lower tone frequencies or higher burst frequencies without compromising the sensitivity of the measurement.